In the decoration of glass articles, in particular panes of glass, glass colorants are used that contain one or more colour-imparting components and one or more glass frits. The glass colorant is stoved after being applied to the substrate, whereby a glass enamel is formed. In the application of glass colorants onto panes of glass the melting behaviour of the glass colorants has to be adapted to the typical conditions of the bending and prestressing process. Typical staving conditions are glass temperatures between about 600 and 650.degree. C. and firing-times of a few minutes. The colour decoration of panes of glass in the architectural and instrument-glass fields necessitates good compatibility of the glass frits contained in the glass colorant with conventional inorganic colouring pigments.
In the field of glass for automobiles in particular, black glass colorants are needed that possess an optimal degree of opacity. Furthermore, in the case of rear windows it has to be possible for the pane of glass that is coated with a glass colorant to be imprinted with a silver conductive pasta for the purpose of forming conducting paths. Lastly, during the stoving process and deformation of the pane any migration of silver through the colorant into the glass should be avoided, since migration results in yellowing of the float glass. A further characteristic that is demanded of glass colorants for the automobile-glass field is that the fused-on colorant should not bond to the pressing tool in the course of the deformation of the pans in accordance with the so-called press-and-bend process.
In many fields of application, including the automobile-glass field, a high resistance to galvano-electrolytes (pH less than 1) is demanded of the stoved glass colorant--that is to say, of the glass enamel, Finally, no lead or cadmium should be contained in the glass frit that is used in the glass colorant.
In the specialist field various lead-free and cadmium-free glass frits and colouring preparations which contain such frits are known for various applications, including the formation of glass enamels on panes of glass for automobiles. Ordinarily, these are frit based on a bismuth/alkali/boron-silicate glass or on an alkali/zinc/boron-silicate glass or mixtures thereof.
Known glass-frit systems satisfy the aforementioned requirements only in part: thus the lead-free and cadmium-free glass frit according to EP-A 0 267 154 is based on a content of Na.sub.2 O, ZnO, B.sub.2 O.sub.3 and SiO.sub.2 and may contain a number of other oxides such as Bi.sub.2 O.sub.3 and Li.sub.2 O. However, as the examples show, only glass frits with a B.sub.2 O.sub.3 or Li.sub.2 O content satisfy the fuse-on range of clearly below 650 C. that is required for application onto glass. A disadvantage of glass frits that contain lithium is that with this process they result in the fracture resistance of panes of glass enamelled therewith being reduced.
In EP-A 0 370 683 a glass flux with 45 to 65 wt-% Bi.sub.2 O.sub.3 and also colouring preparations containing such a frit are described with a view to the production of glass enamels. By the use of such colouring preparations for producing enamelled panes of glass for automobiles, adhesion of the panes imprinted therewith within the bending mould is avoided. This is achieved by means of a crystallization process which sets in after the colorant has been fused on. Although glass frits with a high bismuth content exhibit improved properties, such frits are, depending on the bismuth content, much more expensive than glass frits that have a low bismuth content or that are free from bismuth. In addition, the glass flux also contains 2 to 6 wt-% Li.sub.2 O, resulting in a distinct weakening of the fracture resistance of the enamelled glass, particularly in the case of multiple printing.
A similar way of avoiding the adhesion of coated panes of glass in the course of the press-and-bend process is described by WO 92/0429: the composition which forms an enamel during stoving contains a crystallizable glass frit and nucleating agents based on Zn.sub.2 SiO.sub.4. A disadvantage is that glass enamels based on zinc silicates only have a moderate resistance to acids, so that use in electroplating baths is not possible.
Anti-stick glass colorants such as are described in U.S. Pat. No. 4,959,090 are suitable for use with a view to the production of enamelled bent panes of glass in press-and-bend furnaces. With a view to achieving the anti-stick property the colouring preparations contain metal powder. A disadvantage of the glass enamels produced with these preparations is their inadequate resistance to acids: in addition, a discoloration of the black colours into grey-streaked or brown-streaked colour tones occurs.
The known problem of diffusion of silver from a silver paste which is printed onto the enamel colorant during stoving can be eliminated in accordance with EP-A 0 504 682 by sulphides or polysulphides being added with a view to producing a frit that contains lead. But such enamel colorants contain the unwanted lead. For the same purpose, in accordance with EP-A 0 505 892 lead-free glass frits are disclosed which contain sulphur or sulphides in dissolved form; the frits contain a high proportion of zinc oxide, as a result of which the resistance to acids is again reduced.